Handle additional ServiceNow fields in UI

[justinkambic]

Is your feature request related to a problem? Please describe.
In elastic/kibana#90547 some new fields were added to our ServiceNow alert API object template. These values are hardcoded as null today, but it would be better to allow people to modify them via the UI. The only way to work around this at the moment is to utilize the API directly.

Describe the solution you'd like
Add UI elements to let people modify these fields in the Uptime app.

Describe alternatives you've considered
N/A

Additional context
This is tangentially-related to #294 in that we'll probably be able to resolve them at the same time.

[cnasikas]

The feature was implemented by elastic/kibana#184023. We added an additional_fields field where users can send a JSON payload as they like. @justinkambic Could you confirm that it addresses the customer´s needs, please?